A rotary pendulum of the kind here contemplated comprises an inertial body of large mass suspended from a mounting on the clockwork housing of the timepiece through the intermediary of an elongate element which may be designed as a torsion spring, i.e. a wire or blade of elastic material and negligible mass compared to that of the inertial body, but which could also be a rigid rod rotatably journaled in the clock housing and subjected to a spring force tending to maintain it in a predetermined angular position. In a clock described in a commonly owned U.S. application, Ser. No. 889,435 filed Mar. 23, 1979 by Erich Scheer et al. now abandoned, the pendulum body is rotatably supported on an advantageously nonrotating rod surrounded by a coil spring which is fixedly anchored at its upper end and whose lower end is secured to that body. In all these instances, rotational oscillations of the pendulum about a vertical axis are sustained by undirectional impulses periodically imparted to a lateral extension of the body-supporting element by an impeller which is continuously or quasi-continuously driven by the clockwork, the latter being preferably operated electronically by a crystal-controlled oscillator. According to my aformentioned prior U.S. patent, the impeller may be a cam wheel or disk having a pair of diametrically opposite protuberances alternately engaging a horizontal spur on the torsion spring while that spur rests against a fixed stop. The spur is carried on the torsion spring at a point close to its clamped upper extremity so that the natural frequency of the pendulum is only slightly increased during the half-cycle in which the spur is immobilized by the stop.
In order to protect such a rotary pendulum against accidental damage due to shocks during transportation and handling, various means are known for immobilizing it when the clock is not in use. These precautions, however, are often disregarded by a user who may not even be aware of the existence of such a protective device. According to a prior innovation of mine, described in commonly owned German published specification No. 23, 54 226 filed Oct. 30, 1973, a friction clutch including a coil spring is inserted between the lower extremity of the torsion spring and the inertial body of the pendulum to relieve both longitudinal and torsional overstresses to which the torsion spring may be subjected when the clock is brusquely lifted off a mantelpiece, for example, or when playing children, after removing the protective glass cover usually provided for such timepiece, turn the pendulum body about its axis beyond its normal oscillatory range. That published German publication also teaches the provision of a cup-shaped guide on a base plate underlying the inertial body whose underside has a central stud received with clearance in the cup in order to come to rest on the bottom thereof when the compression of the connecting coil spring reaches a certain degree.
The arrangement just described fulfills its purpose only when the pressure exerted by the coil spring on the friction clutch suffices to prevent any slip between the torsion spring and the inertial body during normal operation but allows such slip to occur when the torque applied to that body approaches a permissible limit representing a maximum twist to which the torsion spring may be safely subjected. Upon prolonged use, however, the clutch faces may wear and the coil spring may fatigue whereby slippage may interfere with the rotary oscillation sought to be sustained.
Since the oscillatory cycle of a rotary pendulum differs as a rule from the recurrence period of the mechanical impulses that can be imparted to the elongate body-supporting element by a clockwork-driven impeller rotating at constant speed, means must be provided for decoupling that impeller from the clockwork whenever its motion is temporarily halted by its engagement with a spur-like extension of that element. In my above-identified prior U.S. patent I have disclosed a transmission including a driving gear and a pinion normally in mesh with each other, the pinion being disengageable from the gear and thus from the clockwork during a phase of an oscillatory cycle in which a tooth of the impeller disk encounters the arrested spur. Such decoupling may also be necessary in the absence of a backstop in order to prevent possible jamming of the clockwork when, for example, the rotary pendulum is willfully arrested in a position in which its spur lies in the orbit of an impeller tooth.